Alternating and repeating blurred and clear areas of subject matter with uniform thin stripes or small polka dots, as seen on the screen. This occurs because the stripes or dots on the subject are spaced slightly differently from the scan lines or pixels on the screen and some of these features line up with the scan lines or pixels while others don't.
A moire pattern will occur in the printing process when two, or more, repeating patterns overlap each other. Similar to the effect on television when a presenter wears a criss-cross pattern on their suit, this effect will result in halftone areas of the print if the line screens of two different inks have been output at the same angle. Therefore, it is important that the screen angles of each printing plate are different. Moire patterns can also occur when a halftone image is scanned and printed, as the dot pattern from the printed halftone can clash with the new line screen. There are numerous ways to reduce this, from blurring the scan slightly, to reducing the size of the scan, to various PhotoShop techniques and filters.